Quick heat treatment of steels

ABSTRACT

An elongated steel workpiece is quickly heat treated in a single heat treatment, by passing it between spaced electrodes and applying a current to the steel between the electrodes thereby to heat it above the austenitic transformation temperature at a heating rate of at least 50* per second, preferably 200* to 100,000*C. per second, holding it at such annealing temperature for at most 300 seconds, preferably 0.04 to 3 seconds, so as to recrystallize the ferrite while dissolving substantially none of the carbides. The annealing temperature is no more than 85% and preferably 50-75% of the melting temperature of the steel in Kelvin degrees. The material is then quenched below the austenitic transformation temperature to prevent austenitic transformation and carbide precipitation.

Prohaszka et al.

[ QUICK HEAT TREATMENT OF STEELS [75] Inventors: Janos Prohaszka; Rudolf Welesz; Andor Mandoki, all of Budapest, Hungary [73] Assignee: Licencia Talalmanyokat Ertekesito Vallalat [22] Filed: Mar. 5, 1973 [21] App]. No.1 337,910

[] Foreign Application Priority Data Mar. 7, 1972 Hungary PO 510 [52] U.S. Cl. 148/12 R, 148/154 [51] Int. Cl C2ld l/ [58] Field of Search C2ld/7/02; 148/12, 154

[56] References Cited UNITED STATES PATENTS 3,271,206 9/1966 Goda et al. 148/154 3,551,216 12/1970 Severing et al. 3,591,427 7/1971 Hansen 148/12 Dec. 17, 1974 3,692,591 9/1972 Dabkowski et al. 148/154 Primary ExaminerW. Stallard Attorney, Agent, or Firm-Young & Thompson [57] ABSTRACT An elongated steel workpiece is quickly heat treated in a single heat treatment, by passing it between spaced electrodes and applying a current to the steel between the electrodes thereby to heat it above the austenitic transformation temperature at a heating rate of at least per second, preferably 200 to 100,000C. per second, holding it at such annealing temperature for at most 300 seconds, preferably 0.04 to 3 seconds, so as to recrystallize the ferrite while dissolving substantially none of the carbides. The annealing temperature is no more than and preferably 50-75% of the melting temperature of the steel in Kelvin degrees. The material is then quenched below the austenitic transformation temperature to prevent austenitic transformation and carbide precipitation.

6 Claims, No Drawings 1 QUICK HEAT TREATMENT OF STEELS The workability by plastic deformation and the mechanical properties needed for the utilization of most finished and semi-finished steel products are provided for by the well-known structure of the steel in which in the relatively soft, plastically readily deformably ferrite, more or less brittle cementites, and other carbide particles and lamellae, respectively, depending on the chemical composition of the steel are to be found, more or less uniformly distributed. During the processing to semi-finished or finished products, these steels are mostly cold worked formed. In order to promote the further cold deformation during the metalworking processes or to provide for the mechanical properties required for the utilization at the end of the operations, these steels must be heat treated. This heat treatment is the so-called furnace annealing consisting of a long soaking and slow cooling, or it is the hardening and tempering, and the patenting, respectively, consisting of the hardening process and the subsequent tempering process.

The furnace annealing is carried out with the steel being in large coils and in an operation requiring high furnace capacity, large working area, intensive material handling and is highly labour-consuming. The hardening and tempering, or the patenting consists of several steps and is an even more intricate process than the furnace annealing, the first step being the relatively long annealing at a high temperature causing often scaling and decarbonization difficulties. The subsequent quicker or slower cooling results eventually in cracks and nearly always in coolant problems. The tempering or soaking following the quick cooling is as lengthy an operation as the furnace annealingThe expense of this intricate heat treatment consisting of several steps is increased by the intricacy and the high investment costs of the required equipment.

The process according to the present invention is suitable for the replacement of both the furnace annealing and the hardening and tempering or patenting by one single short soaking operation at a moderate temperature, in accordance with the structural changes occurring in the steel during the annealing. The new process is based on the better utilization of the structural changes occurring in the steel during the plastic deformation and the subsequent annealing and its essence is as follows: During the processing to finished or semi-finished product, the steel is cold-worked. For the plastic deformation the material must be previously in a suitably soft state. This is ensured by the structure developing therein during the hot plastic deformation preceding the cold deformation and the subsequent slow cooling, respectively, or as a result of a separate furnace-annealing heat treatment. ln this structure the ferrite suitable for plastic deformation is in a soft, readily cold-workable state. Depending on the chemical composition of the steel, the cementites, and other carbides present in the structure are relatively coarse and show an irregular distribution. The cold plastic deformation influences the structure of the steel in three different ways:

1. The ferrite rendering. possible the metalworking,

h ardens. 2. The coarse cementite and carbide particles and lamellae present in the structure break.

3. Under the effect of the considerable plastic deformation of 30 to 80%, the wider ferrite fields located between the cementite and carbide particles and particle groups are flattened to narrow bands so that the great distances between the carbide particles are considerably reduced and the original coarse irregular distribution of the particles is substantially ended.

The hardening of the ferrite, the breaking of the more or less coarse carbide particles and lamellae and their uniform distribution in the structure result in that the tensile strength, the yield point and especially the plasticity of the material are very low so that it cannot be plastically deformed economically any more in this state. Due to the low plasticity, it cannot be sold as a finished product. Before knowing the possibilities provided for by the present invention, when, for the sake of the further plastic deformation or because finished product in an annealed state was required for the utilization, the cold-worked, hardened material was annealed in a furnace, which operation consisted ofa relatively slow heating, of a suitably long soaking at suitably high temperature, dissolving often the eventually present cementite or other carbides and of a slow cooling together with the furnace and during this operation the prevention of the scaling and the elimination of decarbonization in steels of higher carbon content was ensured by a suitable protective atmosphere. During the heat treatment the ferrite was annealed and recrystallized, whereas the cementite and carbide, respectively being eventually dissolved during the soaking precipitating from the solution during the slow cooling, became relatively coarse and lamellar, respectively, having an irregular distribution corresponding to the quantity of perlite and to the sizes of the ferrite particles.

Due to the soft ferrite, this material was highly suitable for further plastic deformation. Due to the coarse, unevenly distributed cementite and perlite, however, the strength indices have unnecessarily shown too low values, hindering thus the increase of strength improving the quality of the finished product achievable during the further processing. The considerable reduction of the strength indices was especially detrimental with materials intended to be sold as finished products in the annealed state. With such materials, namely, the aim would be the maintenance at a high value of the yield point and tensile strength of the material, togeether with as high a plasticity, as possible.

Due to the restricted possibilities of the furnaceannealing, these difficulties could not be disregarded. When the finished product was not needed in the annealed state, but the requirements were for much higher tensile strength, yield point and, as a compromise, a moderate plasticity, elongation and reduction of area, being, however, still enough for the utilization, the heat treating methods applied before the present invention, the hardening and tempering, or the patenting performed this task so that they completely washed out" in the first heat-treating step the properties and structure of the steel developed during the previous technological processes. During the heating to the hardening temperature and the soaking, the hardened ferrite tolerating mostly the plastic deformation is firstly annealed. Then the carbides (primarily the cementite) dissolve. During the subsequent quick cooling, then during the tempering and the patenting soaking, respectively, a completely new structure providing for the mechanical properties meeting the demands develops.

The process according to the present invention is based just on the recognition that for the achievement of the required result such intricate and expensive operations causing great difficulties and containing much risks of failure are not necessary.

it has been found during the elaboration of the process according to the invention that for the achievement of the soft state rendering possible the further plastic deformation forthe softening and recrystallization of the ferrite originally plastic and suitable for metalworking a lengthy furnace-annealing is not necessary, because the said soft state can be achieved during a single, suitably controlled operation consisting of a quick heating-up and a short, possibly a momentary soaking.

The new process solves the above mentioned technical and economical problems of the hardening and tempering or of the patenting in the following manner.

Following the plastic deformation, the steel generally has physical properties approximately meeting the demands of the consumers. Moreover, its strength indices are better, only the plasticity indices are worse. Now, it may seem obvious that, in order to provide for the meeting of solves for the finished products, a heat treatment must be applied not destroying the strength indices but improving to the required extent the plasticity indices. The process according to the invention solves the problem by the heat treatment consisting of a single quick heatingup and a short soaking at a moderate temperature, in the course of which the ferrite rendering possible the metalworking and hardening during the metalworking anneals to the required extent but the brittle cementite or other carbides that are present and broken during the metalworking dissolve to a small extent or do not dissolve at all. The short soaking shall not be followed by all means by quick or slow cooling. By such a heat treatment the plasticity indices, the elongation, the reduction in area of the alloy considerably improve, whereas the strength indices, the tensile strength and the yield point reduce nevertheless, the material can meet the demands of the consumer. These properties can be suitably controlled by the velocity of heating-up, by the temperature and duration of the soaking. Thus, even greater elongation and lower strength, or less elongation and higher strength can also be provided for, if required.

The following Examples show the realization of the process according to the invention.

EXAMPLE 1 1 mm. thick and mm. wide band made of manganese-alloyed steel containing 0.2% carbon, having a C52 quality according to the Hungarian Standard is annealed before rolling since as a consequence of previ ous cold rolling it is too hard and unsuitable for further deformation. The annealing is carried out with the process according to the invention as follows: The coldrolled, smooth, bright band runs from a coil in between two contact rollers of mm width and 200 mm diameter each. The contactrollers are made of tin bronze. The contact surfaces of the rollers are covered with soldered tungsten-silver plates and segments, respectively, containing of silver in order to provide for perfect power transmission and to prevent the risk of burning due to sparking. The rollers press the band surface with a force of about 10 kp. The band running out from between the contact rollers arrives after an about 2000 mm long section in a melted lead bath of about 350C, providing for the electric contact. One pole of a direct-current power source of 40 V is connected to the contact rollers, whereas the other pole is connected to the lead bath. in the 2,000 mm long section between 2000 two contacts a current of about 600 A flows and the Joule heat of the current heats up the band to 700C. The band runs between the two contacts at a speed which makes it possible to cover the 2000 mm distance in 20 secs. Thereof about 2 secs are required until 700C is reached and 18 secs remain for the soaking. Between the two contacts the band is moving between two graphite plates in order to avoid oxidation and decarbonization. The band running out of the contact is guided between lead stripping plates made similarly of graphite and then into a cooling oil bath. The elongation of the band treated in this way is at least 25% rendering possible further plastic deformation.

EXAMPLE 2 The steel band described in Example 1 would be used as finished product with 0.8 X 10 mm. dimensions. The tensile strength of the band previously cold coiled to amounts to 105 kp per sq.mm., its elongation to 2%. The finished product, however, should have a tensile strength of at least kp per sq.mm. and an elongation of 12%. The cold-deformed band is led through the equipment described in Example 1 but in such a manner that the distance between the two contacts is only 200 mm. The voltage is 6 V, the current is 800 A. The running speed of thhe band is the same as in Example 1 so that each point thereof covers the distance between the contacts in 2 seconds. The Joule heat of the current heats'up the band to 760C in about 1 second and the band remains at this temperature for another 1 second. At the end of the operation the material has a tensile strength of to kp per sq.mm. and an elongation of 14 to 18%.

What we claim is:

1. A process for the rapid heat treatment of cold worked steel, comprising heating the steel at the rate of at least 50C. per second to a temperature above the austenitic transformation temperature, holding the steel at said temperature for a time at most 300 seconds to recrystallize the ferrite without substantial dissolution of the carbides present in the steel, and quenching the steel from said temperature to a temperature below the austenitic transformation temperature to prevent the occurence of austenitic transformation and carbide precipitation.

2. A process as claimed in claim 1, in which said heating rate is 200l00,000C. per second.

3. A process as claimed in claim 1, said time being 0.04 to 3 seconds.

4. A process as claimed in claim 1, said temperature being no more than 85% of the melting temperature of the steel measured in Kelvin degrees.

5. A process as claimed in claim 1, said temperature being 5075% of the melting temperature of the steel measured in Kelvin degrees.

6. A process as claimedin claim 1, in which said steel is in the form of an elongated workpiece and said heating is performed by passing said workpiece between and in electrical contact with a spaced pair of electrodes, and passing a current through said workpiece between said electrodes thereby electrically resistively to heat said workpiece. 

1. A PROCESS FOR THE RAPID HEAT TREATMENT OF COLD WORKED STEEL, COMPRISING HEATING THE STEEL AT THE RATE OF AT LEAST 50*C PER SECOND TO A TEMPERATURE ABOVE THE AUSTENITIC TRANSFORMATION TEMPERATURE, HOLDING THE STEEL AT SAID TEMPERATURE FOR A TIME AT MOST 300 SECONDS TO RECRYSTALLIZED THE FERRITE WITHOUT SUBSTANTIAL DISSOLUTION OF THE CARBIDES PRESENT IN THE STEEL, AND QUENCHING THE STEEL FROM SAID TEMPERATURE TO A TEMPERATURE BELOW THE AUSTENITIC TRANSFORMATION TEMPERATURE TO PREVENT THE OCCURENCE OF AUSTENITIC TRANSFORMATION AND CARBODE PRECIPITATION.
 2. A process as claimed in claim 1, in which said heating rate is 200*-100,000*C. per second.
 3. A process as claimed in claim 1, said time being 0.04 to 3 seconds.
 4. A process as claimed in claim 1, said temperature being no more than 85% of the melting temperature of the steel measured in Kelvin degrees.
 5. A process as claimed in claim 1, said temperature being 50-75% of the melting temperature of the steel measured in Kelvin degrees.
 6. A process as claimed in claim 1, in which said steel is in the form of an elongated workpiece and said heating is performed by passing said workpiece between and in electrical contact with a spaced pair of electrodes, and passing a current through said workpiece between said electrodes thereby electrically resistively to heat said workpiece. 